44 Mr. T ; Wedgwood's Experiments and Observations 
collision of the more prominent parts; this, in colourless* 
transparent, and semitransparent bodies, is copiously reflected 
from the whole of the masses, and forms an appearance very 
different from what is ever exhibited by bodies on the heater: 
it may be well illustrated in the upper part of a candle, by re- 
peatedly nearly closing, and quickly opening, the snuffers, 
about the burning wick ; or by sprinkling some powder of 
fluor or marble on a mass of glass heated just under redness. 
Powder of crystal, quartz, agate, &c. is but faintly luminous 
on the heater under redness; accordingly, if the stones them- 
selves be gently rubbed, a faint light is emitted, resembling 
that which their powders give out on the heater. Marble and 
fluor give about the same light on the heater of the tempera- 
ture of 400°, as they do when rubbed ; and probably attrition 
heats some particles in their surfaces to that degree. 
It may, at first, seem an objection to the opinion of heat 
being the cause of the light produced from bodies by attrition, 
that they yield their light when rubbed under water ; but the 
water acts differently to the air, only, as being a stronger con- 
ductor of heat, and can nowise impede the actual generation 
of the heat ; now, as bodies emit their light on the very instant 
of their being heated, the water cannot cool the hot particles 
before they have given their light. 
It is easy to see why bodies emit light instantly when rubbed ; 
for they often send out sparks as soon as the attrition com- 
mences, which proves that particles in their surfaces are in- 
stantly heated to redness by attrition. 
Since hard bodies may be heated to redness by attrition, we 
have an excellent method of discovering the lights they give out 
